Microelectronic, or MEMS, devices often form structures using semiconductor material. Integrated Circuit or (xe2x80x9cICxe2x80x9d) technologies form electronic structure using that same material. Certain applications make it desirable to integrate MEMS devices with integrated circuit electronics on a single substrate.
The prior art has taught making these units in certain ways. A first way processes the IC part first. The MEMS process is then formed onto the semi-finished IC wafer.
The MEMS process must have certain compatibilities, in order to avoid attacking the functionality of an electronic circuit. One primary concern is with operating temperature. All post-IC electronic processes typically need to be done below 400xc2x0 C.: the maximum temperature for aluminum in electronic connections. This eliminates many of the common MEMS structural materials, such as LPCVD silicon nitride and polysilicon deposition; both of which have been typically done above 400xc2x0 C. Therefore, this IC processing system was often used for devices which did not require certain IC dielectrics like polysilicon or metalization layers.
Moreover, many IC foundries do not have processes for mechanical material properties. These foundries focus on producing reliable electronic circuits, as compared with structural processes. As such, these microstructures can have residual stresses and stress gradients. Equally problematic is that the characteristics may vary from run to run.
MEMS devices can form microstructures with various film thicknesses and high aspect ratios. However, the vertical dimensions of the IC layers are often fixed in advance, in order to optimize the IC design. This in turn can limit MEMS design parameters. For example, in order to make free-standing MEMS structures, vertical stress gradients due to the composite nature of the structures can cause curling out of the plane of the substrate.
In a mixed semiconductor/MEMS process, the MEMS processing can be carried out first, or an interweaved process is known in which parts of each are carried out alternately. However, due to the incompatibility of most MEMS materials with the IC processes, this can be difficult.
According to the techniques disclosed herein, more complex, reliable, and economic integrated circuits can be formed by special techniques described herein.